1. Field of the Invention
The invention relates to a beam current measurement device, and to a picture display apparatus comprising such a beam current measurement device.
2. Description of the Related Art
U.S. Pat. No. 4,680,640 (PHN 11,505) discloses a picture display device in which errors occurring in the beam current measurement in a video output stage are reduced by increasing the recharging rate of the capacitive load (constituted by the picture display tube) of a video amplifier with the aid of a bias current or quiescent current which is subtracted again from the current applied to an input of the beam current measuring circuit which is controlled by the video amplifier. More specifically, a cathode of the picture display tube is coupled to the emitter of an emitter-follower drivable by the video amplifier. A negative feedback input of the video amplifier is coupled to an input of the emitter-follower. The collector of the emitter-follower is coupled to an input of the beam current measuring circuit. An anti-blocking circuit is coupled to the cathode of the picture display tube to prevent blocking of the beam current measuring circuit due to leakage currents in the picture display tube. The anti-blocking circuit produces an emitter-follower current of the order of magnitude of at least approximately 100 .mu.A. A current source is coupled to the input of the beam current measuring circuit for draining the emitter-follower current produced by the anti-blocking circuit. In one embodiment, the emitter-follower forms part of a complementary emitter-follower circuit having an additional emitter-follower which is complementary to the first-mentioned emitter-follower, the anti-blocking circuit generates a bias voltage for the basis of the emitter-followers, and the current source is a current mirror circuit an input of which is coupled to an output of a further current mirror circuit an input of which is coupled to the cathode of the additional emitter-follower.
In this embodiment, it appeared that if the actual transfer functions of the current mirrors deviate from their respective nominal transfer functions such that the actual transfer functions are .alpha. and .beta. times the respective nominal transfer functions, the part of the actual cathode current which flows through the current mirrors is affected by the ratio of these deviations .alpha. and .beta., and the measured cathode current is formed by the sum of the part of the actual cathode current which does not flow through the current mirrors and which is thus unaffected, the part of the actual cathode current which flows through the current mirrors and which is affected, and the bias current through the emitter-follower transistors multiplied by a factor depending on .alpha. and .beta.. The bias current through the emitter-followers depends on the manner in which the output amplifier is controlled, and may vary to a large extend and even become zero. This means that in certain circumstances, the distortion in the measured cathode current which is caused by the deviations in the transfer functions may reach values of the order of magnitude of the actual beam current, so that the beam current measurement is seriously affected.